1. Field of the Invention
This invention relates to a control device which, by measuring the elastic deformation induced in the deviation bar by the tension of the fabric under formation and hence extending its action over the entire weaving width, allows effective, accurate and simple automatic control of the warp yarn tension in a loom.
2. Description of the Prior Art
As is well known, in a loom the warp yarn tension must be constantly controlled otherwise it would vary as the delivery of said yarn by the beam proceeds, because the radius of the yarn wound on the beam decreases whereas the angular velocity of this latter remains constant, the tangential delivery speed of the warp yarn hence decreasing, with consequent increase in the yarn tension.
In the current state of the art various types of devices are already known for controlling the warp yarn tension. With the most commonly used type the loom yarn holder is mounted on a rocker arm and opposed by a spring which is loaded to an extent representing the desired tension for the warp yarn, the inclination of said rocker arm to its normal working position being determined by a proximity sensor which drives the geared operating motor for said beam. In this manner indirect control of the warp tension is achieved by inclining said rocker arm to its normal working position.
Such a known device is of simple low-cost construction but has the drawback of not being able to maintain the warp yarn tension constant when the rocker am remains in its normal working position, following the different angular attitude which the warp yarn assumes as the yarn is finishing on the beam.
To eliminate this drawback a fixed deviator roller is generally positioned in the path of the warp yarn between the beam and the yarn holding roller, resulting in a constant warp yarn direction towards the yarn holding roller as the inclination of said warp varies due to the continuous depletion of the yarn package on the beam.
In addition to an obviously greater cost, this method has however the drawback common to all elastic mechanical systems, ie of having an inherent resonance frequency and an inherent inertia and of being subject to friction, which by introducing phasing errors and hence delays in intervention, results in deleterious abnormal excess tension in the warp yarn both during loom starting and during normal working.
A control device is also known which does not suffer from these drawbacks. In this device the warp yarn tension is controlled by a tension sensor supported by a fixed deviation bar for the fabric under formation and directly measuring the tension in the fabric, this evidently being the same as the warp yarn tension given that this yarn is connected to the fabric.
This device suffers however from the drawback that current tension sensors are of such dimensions as to be able to measure only the average tension in a small band of the fabric, so that the measurement cannot be sufficiently representative of the actual tension to which the entire width of the fabric and hence the entire warp yarn is effectively exposed.